Default values for properties based on datetime fields are not schema-conformant

Overview

If you have an SDC with a date prop, Experience Builder will mistakenly create a Component config entity for it that doesn't comply with config schema. The component entity will have a default_value: { value: 'whatever' }} structure, which doesn't match the field.value.datetime config schema type from core.

Proposed resolution

@larowlan already went down this rabbit hole and discovered that -- strange as this (and it really is) -- core's doing the right thing here. Date fields use a relative default value that isn't schema-defined.

XB needs to match that by calling \Drupal\Core\Field\FieldItemListInterface::processDefaultValue() on the default values it generates for prop definitions in Component entities. It's forgetting to do that right now, which is why it runs into this bug.

User interface changes

None.